Various methods are proposed, each of which reconstructs various images from a digital image produced by image capturing and outputs them. For example, a plenoptic camera proposed in “Light Field Photography with a Hand-held Plenoptic Camera” (hereinafter referred to as NPL 1) uses a light field photography technique to obtain information on positions and angles of light rays from an object, that is, light field information.
Description will be made of the plenoptic camera with reference to FIG. 1. The plenoptic camera is provided with a microlens array 101c disposed at an imaging position of an image capturing optical system 101 that is constituted by a main lens group 101b and an aperture stop 101a, and with an image sensor 102 disposed at a rear of the microlens array 101c. The microlens array 101c serves as a separator to prevent light rays passing a certain point A in an object space from mixing with light rays passing a point near the point A on the image sensor 102. As understood from FIG. 1, an upper ray, a principal pay and a lower ray from the point A are respectively received by mutually different pixels on the image sensor 102. Therefore, the image sensor 102 can photoelectrically convert the light rays passing the point A and separated according to their angles.
On the other hand, “Full Resolution Light Field Rendering” (hereinafter referred to as NPL 2) proposes another method of acquiring the light field information, which is shown in FIGS. 2 and 3. Constituent elements shown in FIGS. 2 and 3 are same as those shown in FIG. 1. In FIG. 2, the microlens array 101c disposed at a rear of the imaging position of the main lens group 101b causes the light rays passing the point A to reform images on the image sensor 102, which makes it possible to acquire (photoelectrically convert) the light rays passing the point A and separated according to their angles. In FIG. 3, the microlens array 101c disposed at a front of the imaging position of the main lens group 101b causes the light rays passing the point A to form images on the image sensor 102, which makes it possible to acquire the light rays passing the point A and separated according to their angles. The microlens array 101c shown in FIG. 2 reforms a real image of the main lens group 101b, and on the other hand, the microlens array 10c shown in FIG. 3 forms a virtual image of the main lens group 10b. 
The plenoptic camera proposed in NPL 1 and the method proposed in NPL 2 are different from each other in principle of acquisition of the light field information. However, the light field information acquired by each of these methods makes it possible to reconstruct, by using image processing, an image corresponding to an arbitrary focus position or an image viewed from an arbitrary viewing direction.
Moreover, Japanese Patent Laid-Open No. 2009-105717 (hereinafter referred to as “PTL 1”) discloses a method for suppressing generation of aberration in a microlens array, which sets a focal length of the microlens to be smaller than a product of twice a depth resolution required for an image pickup apparatus and a lateral magnification of an image capturing optical system.